Conventionally, a steering wheel method and a joystick lever method have been known as steering methods of construction vehicles. In the steering wheel method, an operator turns a steering wheel provided at a driver's seat, thereby obtaining a steering angle almost proportional to the turning angle, and the maximum turning angle is, for example, two or three turns respectively to the right and left. According to the steering wheel method, a steering angle corresponding to a turning angle is obtained, therefore enabling a fine steering operation. In addition, a construction vehicle can be driven with the same feeling as in driving a car, which makes it easy to travel.
Meanwhile, in the joystick lever method, an operator tilts a joystick lever provided at a driver's seat, thereby obtaining a steering speed (changing speed of a steering angle) which is almost proportional to the tilting angle. The maximum tilting angle is the maximum angle of turning the wrist respectively to the right and left, thereby enabling an operation with one hand. Consequently, it is easy to travel while operating a working machine.
The conventional steering methods as described above, however, have the following disadvantages. In the steering wheel method, a larger turning angle is required in order to obtain a larger steering angle, which becomes one of the reasons for causing fatigue of the operator in a construction vehicle requiring frequent switching of the steering. When traveling during an operation, it is necessary to operate a steering wheel with one hand, which causes the disadvantage of the steering operation being very difficult.
In the joystick method, a steering speed almost proportional to the tilting angle of a joystick lever is obtained, and therefore in a situation in which a lever is tilted, the steering is turned too much. For this reason, each time when a steering angle reaches a target steering angle in one steering operation, it is necessary to return the joystick lever to a neutral position so that the steering speed recovers to 0 (zero). In addition, when the steering angle is further returned to the straight-ahead position, it is necessary to carry out the same operation as above in the reverse direction. Accordingly, the number of operations increases, thereby increasing the workload on the operator. Further, in order to operate the steering at a minute steering angle, it is necessary to continuously operate the lever in a minute tilting angle range in the reverse directions in a short time, therefore making the operation extremely difficult, and the steering tends to be unstable.